The invention relates to a method for preparing a Stein Hall adhesive, comprising the steps:                preparing a carrier portion by gelatinising at least a part of the total starch in the adhesive in an alkaline medium;        diluting the carrier portion with a volume of water. and        adding the remaining secundary starch,wherein in a first step said carrier portion is prepared using part of the alkali, and in a second step the carrier portion is diluted with a volume of water, followed by the addition of at least a portion of the secondary starch.        
The invention further relates to a high dry substance Stein Hall adhesive (also called a high concentration Stein Hall adhesive or glue) obtained by a such method.
Starch based adhesives are well known in the corrugated board industry. Among the adhesives used, Stein Hall-type adhesives are the most popular.
Stein Hall adhesives are generally known as comprising a carrier portion of the adhesive that is obtained by gelatinising about 10–20% of the total starch in the glue using caustic soda. The resultant paste is then cooled and diluted with water and the remaining starch is then added as native granular starch. Also borax (1,5–2,5%) is added.
Under these circumstances, the maximum total starch concentration in these Stein Hall-type formulations does not surpass 25%, although values up to 30–37% on dry weight basis have been reported. These higher values were obtained when viscosity of the carrier was reduced by means of an oxidation reaction or the like.
The disadvantage of these low-concentration adhesives is that they contain higher water contents, reduce adhesion speed and retard drying of the adhesive layer in the adhesion step. In addition higher dry substance adhesives result in reduced energy consumption because smaller quantities of water need to be evaporated.
Subsequently, higher starch concentrations in the glue are desirable. But, it is also known that, when starch is used as the binding agent, there is a limit to the highest amount of starch that can be used. At higher amounts of starch the amount of water becomes insufficient to guarantee the desired adhesion, while the viscosity of the adhesive becomes too high.
High concentration starch based adhesives have already been described in the prior art. In EP 0 038 627 a starch based adhesive is described that is composed of a completely gelatinised starch fraction and a partly swollen starch fraction, this in a ratio between 0,2:100 and 2:1. The adhesive may contain between 10% and 40% dry weight starch solids. The gelatinised fraction is obtained by heat treatment at 95° C. or higher, and not via mixing with concentrated alkali as in a classic Stein Hall formulation. In fact, it is a no-carrier type adhesive to which a gelatinised starch fraction is added to improve wet tack of the adhesive.
In EP 0 576 221 adhesive compositions are disclosed comprising from 15 to 35% by weight starch solids. A substantial quantity of the starch is partially swollen (preferably between 20 and 50%).
EP 0 376 301 describes an adhesive prepared by treating a starch slurry in a jet cooker at a temperature between 120 and 200° C. to obtain a gelatinised dispersion. The ungelatinised is then added to this gelatinised dispersion. The total starch solids content in this formula, as illustrated in the examples, varied between 32 and 40%.
Also in GB 1 571 336, a method is described for the preparation of a starch-based aqueous adhesive. In this method, a carrier portion is prepared of gelatinised molecularly dispersed starch at a solids content from 3% to 40% by weight by cooing a starch slurry using a jet cooker. The carrier portion is ten mixed with a slurry of ungelatinize starch. Alkali is provided in the resulting mixture.
The adhesives described in these applications are not typical Stein Hall compositions.
In EP 1 101 809, high dry substance starch-based adhesives are described containing at least 40%, primarily at least 46% dry substance. This high dry substance content is realised by the addition of inorganic salts, preferably calcium carbonate, to the starch-based adhesive. The starch/filler ratio is preferably at least 60:40. The adhesive can be of the Stein-Hall, nosier or Minocar-type.
In EP 0 409 240, a high concentration starch slurry adhesive is described comprising between 20 and 60% by weight of starch solids. This adhesive is stabilised through the addition of a gum such as xanthan or rhamsan gum and some sodium hydroxide.
In WO 9850478 high dry substance adhesive compositions are that contain a substantive amount of water-soluble alkaline silicate, together with starch, borax and sodium hydroxide. Adhesive compositions containing up to 45% solids are disclosed in the examples.
Part of the above-cited prior-art documents are not related to Stein-Hall type adhesives, such as EP 0 038 627, EP 0 576 221, EP 0 376 301 and EP 0 409 240.
The two other documents cited, i.e. EP 1 101 809 and WO 9850478, may comprise Stein Hall-type adhesives, but they differ substantially from standard formulations in that they contain inorganic filler material that contributes to the high dry substance of the glue.
In the state of the art, further high dry substance Stein Hall-type adhesives have been disclosed.
In U.S. Pat. No. 3,912,531 high dry substance adhesives of the Stein Hall-type are disclosed containing up to 40% by weight of starch solids.
However, the disadvantage of this kind of high dry substance Stein Hall adhesives is that this is realised by using a fluidity starch having a fluidity between 25 and 90 as the carrier starch or as part of the carrier starch.
Also EP 0 049 009 discloses high dry substance glues of the Stein Hall-type. The problem underlying this patent document is how to obtain a final high DS glue where the viscosity does not increase strongly and whereby the mixture remains pumpable. In fact the glue must show some storage stability and workable viscosity.
In the patent document, it is stated that high dry substance glues can not be obtained with the conventional technology for preparing Stein Hall-type adhesives (which have a low dry substance). As described above, the method by which these kinds of Stein Hall-type glues are prepared comprises preparing the carrier and thereafter diluting it by lowering the alkalinity. When the alkalinity would not be lowered, the addition of the secundary starch would be affected and would result in granule swelling and viscosity instability.
However, if high dry substance clues have to be obtained, dilution is becoming insufficient to prevent undesirable swelling. In addition to this, the combination of less diluted carrier and high secundary starch concentration will result in composition that are no longer workable.
The solution to this problem that is proposed in the application EP 0 049 009 consists in oxidising the Wrier portion and then gelatinising the oxidised starch in presence of the necessary amount of sodium hydroxide, thereby reducing the viscosity of the carrier to a value which makes it possible to add the required secundary starch. This secundary starch is then added to reach a 40% d.s. glue. It should be emphasised that the carrier starch portion in the glue represents more than 14% of the starch dry substance present in the glue.
The disadvantage of this method is that high dry substance glues are obtained with the necessity of using reduced viscosity starches in the carrier fraction.
The reduced viscosity starches referred to in the patent documents of the state of the art typically are fluidity starches or oxidised starches in which the fluidity starches are obtained via chemical or enzymatic degradation.
In U.S. Pat. No. 6,048,391, a process is described for the preparation of a starch based glue for making corrugated board. The process is carried out in a system which includes a first and relatively large mixing vessel containing a stirrer. A second and smaller mixing vessel is connected to the first mixing vessel via a recirculation path. The second and smaller mixing vessel contains means for exerting a shearing force which is greater than the shearing force exerted in the first and larger mixing vessel. The process is carried out in two steps. In a first step, water and starch are supplied to form a carrier in the first mixing vessel and then the resulting carrier is transported to the second mixing vessel. Lye is first added in a controlled manner in the second mixing vessel to prepare the carrier. A second step adds farther amount of starch, water, borax, and a second shot of lye to the carrier while in the small vessel.
The disadvantage of this system is that a supplementary mixing vessel is provided, wherein a circulation loop is introduced. In this way, a technical more complex and more expensive system is provided. Furthermore, the Stein Hall adhesives as obtained by this method do not have a high dry substance.